This invention relates to an electrophotography printer such as a laser beam printer and, more particularly, to a drum driving unit of this kind of printer.
A drum driving unit of a conventional electrophotography printer will be described below with reference to FIG. 2. An end bracket 2 is attached to an end of a drum 1 constituted by a non-magnetic cylindrical body having an outer circumferential surface to which a photosensitive material is applied. An end bracket shaft 3 is attached to the end bracket 2 coaxially with the same. A spring type one-way clutch 5 is coaxially incorporated in a drum shaft 4 for transmitting a driving torque to the drum 1. The drum 1 is an expendable member and needs to be changed periodically. To change the drum 1, a drum assembly 6 integrally formed the drum 1, the end bracket 2 and the end bracket shaft 3 is changed. The end bracket shaft 3 of a new drum 1 is brought into engagement with the one-way clutch 5. In ordinary electrophotography printers, the drum 1 is rotated in only one direction. The arrangement may therefore be such that the one-way clutch 5 is locked with respect to the direction of rotation of the drum 1 to enable the torque of the drum shaft 4 to be transmitted to the drum assembly 6. At the time of of the exchange, the attachment position of the drum 1 in the rotational direction is not particularly limited, and it is sufficient to establish engagement between the end bracket shaft 3 and the one-way clutch 5 incorporated in the drum shaft 4.
The above-described conventional art entails a problem described below.
FIG. 3 shows the results of an actual test which was conducted in such a manner that an optical encoder was attached to the drum of a conventional type of printer to detect the rotational speed of the drum and, hence, to measure the variation in the rotational speed (unevenness) with respect to a reference speed during one revolution of the drum. In FIG. 3, segments B which indicate abnormally large speed variations are clearly seen between segments A which indicate speeds in a normal range. Variations in the drum speed are caused by the variation in the force of friction between the drum and a blade cleaner for removing surplus part of the ink applied to the drum, but deteriorations of the image qualities caused by the influence of these variations are small. Therefore the present invention is not intended to remove small speed variations such as those indicated by the segments A. FIG. 4 schematically shows features of an actual image sample printed on the condition shown in FIG. 3. This actual image sample is called a half tone image formed by drawing lateral lines with small regular pitches to obtain a half tone. Since the recent market trend is being shifted from setting importance on characters to setting importance on graphics, the subject of how to output half tone images having improved qualities is important. In the conventional printers, however, the printing pitches in the direction of rotation of the drum is made uneven due to unevenness of the rotation of the drum 1 (speed variations), resulting in the formation of a light-shade striped pattern, such as that shown in FIG. 4. This phenomenon considerably deteriorates the image qualities. It is therefore very important to prevent this phenomenon.
The reason for the occurrence of such large variations in the rotational speed is that the one-way clutch is repeatedly locked and released irregularly by small vibrations and variations in the load, since the one-way clutch has a restraining force with respect to rotation in only one direction and has no restraining force in the opposite direction.
Japanese Utility Model Unexamined Publication No. 1-75274 discloses a construction in which the entire drum is attached with a pressing force of a spring. However, it is not possible to attain an object of the present invention, i.e., the object of preventing unevenness of the drum rotation and, hence, light-shade unevenness of a half tone by only simply pressing the drum.
Japanese Patent Unexamined Publication No. 58-193931 discloses a method relating to a shaft joint, in which a member constituted by an elastic body having a taper groove is attached to a groove portion of a shaft, and this groove portion is engaged with a spring pin. For driving a laser printer drum, however, it is not suitable to apply the construction in which an elastic member is used for a shaft joint, because the elastic member is elastically deformed with changes in the load to cause vibrations in the axial direction, resulting in light-shade unevenness in the printed image.